This invention relates generally to radio communication systems, and more particularly to a repeater for use when a subscriber unit is not in communication with a central station as in trunked radio frequency communication systems.
As disclosed in U.S. Pat. No. 4,723,264, to establish communication between trunked subscriber units, a requesting subscriber unit transmits a call request to a central station (or central controller). Upon receipt of the request, the central station returns a channel grant code (if a channel is available), which instructs the requesting subscriber unit to move to a specified communication channel as a transmitting unit, while causing all called subscriber units to move to the same communication channel as receiving units.
Generally, trunked communication channels comprise a pair of frequencies. An inbound frequency carries information from the subscriber units to the central station, while the outbound frequency carries information from the central station to the subscriber units. Subscriber units may be portable, mobile or fixed. A portable unit is typically understood to be a transceiving device designed to be carried on or about the person. A mobile is a transceiving device designed to be installed in vehicles. A fixed station is typically a permanent or semi-permanent installation in a building or another fixed site. As used herein, all of these transceiving devices are collectively referred to as subscriber units.
One situation where a subscriber unit is not in communication with a central station occurs when the subscriber unit is operating at such a distance (or range) from the central station that the subscriber unit has insufficient power to transmit to the central station; even though the more powerful central station's tansmission can reach the subscriber unit. In this area of weak signal strength, the subscriber unit is out-of-range. Usually, the weak signal strength zone is located in a remote or rural area, where there are very few subscribers present to utilize the system. Therefore, it is not economically to install a conventional repeater or another central station.
One approach is to provide more than one conventional repeater at each weak signal strength area in a simulcast system to amplify all the channels in the coverage area. However, simulcast overlap problems such as intermodulation interferences may occur.
As illustrated in U.S. Pat. No. 4,677,687, one solution pertaining to mobile units involves installing or operating a repeater between the central station and the mobile units to selectively relay information on a booster channel. When the mobile unit detects a weak signal, the mobile unit automatically switches to a predetermined booster channel for communication with the repeater, which will relay the communication to the central station and (and vice versa). This approach requires that each of the mobile units be capable of recognizing the necessity to switch to the booster channel. Generally, for cost efficiency, intalling an economical repeater is preferred over recalling and changing existing units in the field.
Even though the central station generally has more power to tramsmit than the subscriber unit, beyond a certain distance at a fixed power level, the central station will have insufficient power to reach the subscriber unit, especially if the unit roams farther out-of-range. Here, the central station has insufficient communication range for efficient system operation. Therefore, an inexpensive roaming coverage may be needed in some systems as in a trunked specialized mobile radio (SMR) system. Likewise for a system where the central station is located far away from a building, it is desirable to extend the distance the central station can transmit to facilitate communication coverage anywhere within the building.